Krebs Cycle Diagram

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The steps in the oxidation of pyruvic acid were traced by the British biochemist, Hans Krebs in 1937. The first compound formed in tricarboxylic acid cycle is citric acid; hence this reaction is also called citric acid cycle.

The Krebs cycle begins with acetyl CoA.

1) In the first reaction of citric acid cycle, one molecule of acetyl CoA combines with 4 carbon oxalo acetic
acid to form citric acid . This reaction is catalyzed by an enzyme citrate synthase. The reaction utilizes one
molecule of water and releases CoA.

2) Citric acid is then isomerized to isocitric acid with a molecule of water.

3) Isocitric acid is converted to oxalo succinic acid. Here NAD+ is reduced to NADH+ H+.

4) Oxalo succinic acid is then decarboxylated to form a 5 carbon compound, alpha ketoglutaric acid.

5) Alpha Ketoglutaric acid is decarboxylated to form succinyl CoA. It is a dehydrogenation reaction. Here
NAD+ is reduced to NADH + H+. CoA is required in this reaction.

6) Succinyl CoA loses its CoA and reacts with glyceraldehyde diphosphate to form succinic acid and
glyceraldehyde triphosphate. Later GTP transfers its one phosphate to ADP and thus ATP is formed.

7) Succinic acid is converted to fumaric acid. It is a dehydrogenation reaction. Here FAD is reduced to
FADH2.

8) A water molecule is added to fumaric acid to form malic acid.

9) Finally malic acid is converted into oxalo acetic acid. Here NAD+ is reduced to NADH + H+.
Oxalo acetic acid becomes available to combine with acetyl CoA to start a new cycle.

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